The present invention relates a focus control apparatus to be mounted in an optical apparatus, such as a camera.
A contrast autofocus (“AF”) system among the AF control for use with a camera, etc. uses an image-pickup device for an image-taking optical system, and detects a high frequency component in a brightness signal obtained from the image-pickup device or a component of the contrast state (visibility) (referred to as an “AF evaluation value” hereinafter). The contrast AF system thereby provides highly precise, small and inexpensive AF control since no focal-point detecting system is required.
Known as the contrast AF system are a full scan system that scans from infinity to a close-up end, detects an AF evaluation value, and moves a lens to a (focal) position corresponding to the maximum AF evaluation value, and a contrast detection system that moves a lens by a predetermined amount in a direction for increasing the AF evaluation value, and for detecting the maximum AF evaluation value (or a peak).
Other known AF systems include a phase-difference detecting AF system and a triangulation AF system, each of which uses a sensor different from an image-pickup device in the image-taking optical system to form a dedicated detection system. Problematically, these focus control systems have focal point detection accuracy lower than the contrast AF system because they use a dedicated detecting system subject to time and temperature variations, and cause a parallax between a subject and a distance-measuring position depending upon the distance.
These phase-difference detecting AF system and triangulation AF system calculate a driving amount of a lens up to a focal position based on the detected phase difference and distance. They do not require scanning or peak detections by actually driving a lens in detecting a focal position as in the contrast AF system, and have an advantage in a fast focal-point detecting speed.
Japanese Patent Application, Publication No. 2001-255456 discloses a hybrid AF control system that combines the contrast AF system with the triangulation AF system (or phase-difference detecting AF system) so as to take advantage of these systems.
For example, the hybrid AF system selects the contrast AF system for a static or stationary subject that does not change a distance, and the triangulation AF system for a dynamic or moving subject that changes the distance, attempting to detect a focal point for both the dynamic and static subjects.
However, the hybrid AF proposed in Japanese Patent Application, Publication No. 2001-255456 completely switches the AF system and has a problem in that it cannot maintain the sufficient focal-point detecting accuracy for a distance-changing subject. In other words, while the contrast AF system can maintain the sufficient accuracy for a stationary subject, the triangulation AF system exclusively used for a dynamic or distance-changing subject is greatly subject to an erroneous measurement of a focal position due to the time and temperature variations, and causes a high pixel density image-pickup device an out-of-focus state, for example.
Japanese Patent Application, Publication No. 2001-264622 also discloses the hybrid AF system that switches focus control between an outer-measuring triangulation AF system (or passive AF system) and a TTL contrast AF system in accordance with an operation of an image-taking switch. It detects a focal point with both AF systems and provides such control as selects one of the systems based on two focal-position measurement results.
Japanese Patent Application, Publication No. 2001-141984 discloses a hybrid AF system that switches focus control to the contrast AF system after driving a lens to a front-pin or back-pin position by a predetermined amount away from a focal point detected by the outer-measuring triangulation AF system (active AF system). This system returns to the triangulation AF system if it cannot detect the contrast after switching.
Japanese Patent Application, Publication No. 3-81713 discloses focus control that identifies a focal position direction and controls a focus driving speed using the triangulation AF system, and switches to the contrast AF system when it detects the AF evaluation value.
However, the hybrid AF system disclosed in Japanese Patent Application, Publication No. 2001-141984 causes two AF systems to detect focal points for different subjects, for example, due to the parallax. When the contrast AF system cannot sufficiently detect a focal point after the triangulation AF system detects the focal point, the mode is again switched to the triangulation AF system and the focal position is possibly unavailable due to the temporary hunting.
In addition, the hybrid AF system disclosed in Japanese Patent Application, Publication No. 3-81713 uniformly switches two AF systems based only on the AF evaluation value that is greatly variable depending upon a subject. A switching position is sometimes far from a focal position and sometimes close to it in the AF system, resulting in an unstable focus control.